I have the same issue, its the sidewinder mouse. My PC is clean, no nothing. I do not have the issue with the same mouse "backup", the problem sounds like failing hardware.
This guy made a program to fix it, so far it has worked. But has only passed 40 minutes. Hope it helps, also I do not see it aiding long term but who knows. So improvement!
Mouse Fix section at bottom of page.http://www.danieljackson.co.uk/fun/old/